T-Powering for Condenser Microphones
Copyright 1997 by Rick Chinn. All rights reserved.
T-powering, (Tonader powering) sometimes known as A-B powering, technically known as Modulation Lead Powering is a non-compatible method of remote powering a condenser microphone. It is considered non-compatible because microphone outlets so equipped may never be used with non-t-powered microphones, which includes dynamic and ribbon microphones.
In a general way: T-powering uses the two modulation (signal) leads of the microphone for simultaneous powering. Both the microhone's amplifiers as well as the microphone input must be DC blocked from the microphone powering voltage. The microphone line is still balanced.
There are two general ways to supply power in this scheme.
- DC block the input from the microphone line. Apply the powering voltage from the 'battery' to one side of the mic line via a 180-ohm resistor. Connect the other side of the mic line to battery minus via another 180-ohm resistor. Sometimes the 'battery' is ground referenced, and then the second 180-ohm resistor connects to ground.
- The microphone input has an input transformer with a split primary winding. The start of one winding and the finish of the other go to the input connector. The finish of the first winding and the start of the 2nd winding are connected together via a large-valued electrolytic capacitor. The DC voltages are applied via 180 ohm resistors across the capacitor. It is imperative that the DC supply be very pure, and low-noise.
There is no polarity standard for the powering signal. That is why Nagra tape recorders have both positive T-12 and negative T12 powering. The powering voltage varies with the microphone. As you can see, this system is not compatible with anything other than itself.
Plugging dynamic, ribbon or phantom powered microphones into a T-powered outlet is an invitation to disaster.
- The series resistance serves to protect a dynamic microphone from harm but if there is an output transformer, its core will probably be magnetized, which affects its distortion performance.
- A ribbon mic would probably be harmed.
- A carbon microphone would probably work.
DIN standard 45 595 defines this method.
Links and Schematics
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Last modified 04/14/2011.
18:27:16
